Known optical, incremental position measuring instruments include not only one or more incremental divisions on a scale and scanning unit but, typically, also include one or more fields with reference marks or reference mark structures for generating reference pulse signals. The reference pulse signals are used for unambiguous identification of defined relative positions of two objects movable in relation to one another whose relative motion is intended to be detected with the aid of the optical position measuring instrument. For example, the objects that are movable in relation to one another may be a workpiece and a tool of a numerically controlled machine tool.
Typically, the reference mark structure on the scale and scanning unit include a number of alternatingly disposed regions having optically different properties. Depending on whether the position measuring instrument is operated with transmitted light or reflected light, the alternatingly disposed regions are either optically transparent and opaque, or reflective and nonreflective. Typically, both on the scale and on the scanning unit, identical periodic structures are disposed in a reference mark field and the reference mark field is illuminated with a collimated beam. Each transparent or non-reflective region on the scale is assigned a transparent or non-reflective region respectfully on the scanning unit, at exactly the same corresponding relative position. German Patent Disclosure No. DE 18 14 785, assigned to the present assignee and incorporated herein by reference discloses such periodic structures.
U.S. Pat. No. 5,065,017, for example, discloses optimized reference mark structures on the scale and scanning unit so that at a defined relative position of the scale and scanning unit, a pronounced maximum signal results, while secondary maximums are suppressed as much as possible. This is achieved by certain aperiodic distributions of the regions with different optical properties. Both on the scale and the scanner, however, identical distributions of the corresponding regions are provided for that purpose.
In addition, with respect to the optical position measuring instruments discussed thus far in which the scale and scanner divisions are illuminated by a collimated beam, optical position measuring instruments utilizing so-called divergent illumination are also known. In such position measuring instruments, voluminous collimation optics are not needed between the light source and the first division structure through which the beams pass, and thus, in particular, a compact design of the optical position measuring instrument is possible. U.S. Pat. No. 4,823,001 discloses such an example.
In these optical position measuring instruments as well, it is desirable to be able to generate reference pulse signals with regard to certain relative positions of the scale and the scanning unit. No suggestion, however, of generating a reference pulse signal is mentioned in this patent.
The aforementioned position measuring instruments, however, are unsuitable for generating a reference pulse signal utilizing divergent illumination. In these instruments, the dimensioning of the corresponding structures on the scale and scanner are always based on a collimated beam that illuminates the structures.